1. Field of the Invention
The invention pertains to a guide for the piston rod of a piston-cylinder assembly, especially for a level-control element operating with internal pressure, where a guide body holds the cylinder tube and the outside tube and simultaneously centers the piston rod in its central hole. The guide body has at least two axially spaced sealing elements, which rest on the outside surface of the piston rod and seal off the interior space of the piston-cylinder unit from the atmosphere.
2. Description of the Related Art
Guides and seals of this type for the piston rods of hydraulic, pneumatic, and hydropneumatic assemblies are already known (DE 198 13 240). This guide is provided with at least one piston rod seal, where the piston rod, which is usually connected to a damping piston, is centered in the cylinder by this guide and sealed but is guided with freedom of axial movement.
Self-pumping hydropneumatic shock-absorbing struts with internal level control are also known, in which the damping medium-filled working cylinders are under the pressure of at least one gas cushion provided in a high-pressure chamber and acting as a spring. In piston-cylinder assemblies in which the working space is under pressure, the problem has been encountered that the installation space available in motor vehicles is so limited that only piston-cylinder assemblies with very small housing diameters and thus also with small piston rod diameters can be used. To provide the same performance capacity, the internal operating pressure of the piston-cylinder assembly must therefore be considerably increased. The essential weak point in piston-cylinder assemblies such as this is the sealing of the piston rod, which is able to move axially in the guide. The previously known seals no longer have adequate service lives at the required operating pressures of approximately 50-150 bars; the increased coefficients of friction of the seal against the surface of the piston rod arising from the high internal pressure also impair the smoothness of the ride. This is not tolerated by automobile manufacturers.
It is therefore the object of the present invention to improve the piston rod guide with an appropriate piston rod seal in such a way that a sufficiently leak-proof and durable seal is created, even at high operating pressures in the interior of the piston-cylinder assembly.
To achieve this object, a flow connection extends between the intermediate space between the two high-pressure seals and the interior space of the assembly.
According to an essential feature, it is provided that the flow connection is provided with a nonreturn valve, through which a flow can proceed toward the interior space.
Additional elaborations provide that, in the case of a level-control element operating with internal pressure, the flow connections lead to a low-pressure tank space or to the high-pressure working space.
In the case of designs in which the intermediate space between the seals is connected to the high-pressure working space, the nonreturn valve blocks the flow in the direction of the intermediate space between the seals.
For friction and wear reasons, the primary seal is made to allow leakage into the intermediate space between the seals. The pressure rises as a result of the leaking oil and, during the operation of the vehicle, is decreased again via the flow connection and the nonreturn valve at the baseline pressure minus the pressure reductions which result in the cylinder tube at high pressure damping velocities. As a result, the secondary seal can be designed for low friction and low wear at lower pressures.
In designs in which the intermediate space between the seals is connected to the low-pressure space of the piston-cylinder assembly, a nonreturn valve is also installed in this flow connection to block the flow toward the intermediate space. In this design, too, the primary seal is made for friction and wear reasons in such a way with respect to form and material that leakage is possible into the intermediate space between the seals. The pressure, which rises as a result of the leaking oil, is decreased during the operation of the vehicle via the flow connection and the nonreturn valve preferably in the upward-adjusted state (baseline pressure minus reduction caused by the transport of oil). As a result of this measure, too, the secondary seal can be designed for low friction and low wear at lower pressures.
In accordance with an embodiment which is easy to manufacture, it is provided that the nonreturn valve consists of a valve seat provided in the end surface of the guide and an elastic valve disk, which is installed at the end of the flow connection leading to the interior space.
In another design, the nonreturn valve is introduced as a modular unit into the flow connection.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.